Coating slips based on partially soluble dextrins of high molecular weight

ABSTRACT

A coating slip includes water, at least one pigment and at least one modified starch, exhibiting: a weight-average molecular weight ranging from 250 000 to 2 000 000 Da; and a solubility, measured according to a test A, ranging from 50 to 85%. A process for the preparation of the slip, and a process for coating paper using this slip are also described.

A subject-matter of the invention is novel coating compositions usefulin the manufacture of coated paper comprising a novel modified starch ofprecise molecular weight and precise solubility. The invention alsorelates to a process for the manufacture of coated paper from this novelcomposition and to the coated paper thus obtained.

In the paper and board industry, numerous different grades of productsexist. Paper and board of high quality, for example those used inmagazines or information leaflets, are generally coated papers orboards, combined together under the term “coated papers” in the presentpatent application.

A coated paper is a paper, the surface of which is covered with one ormore “layers”, generally based on pigments mixed with binders andvarious additional products.

These layers are applied by a coating process which is carried out usingaqueous compositions known as “coating colours”. The aim of thisoperation is to render the surface of the paper, which is rough andmacroporous in the absence of treatment, even and microporous, in orderto make possible better reproduction of the printings.

Furthermore, it is possible, by this operation, to improve thewhiteness, the gloss or the feel of the printed paper.

Coated paper also has to exhibit a good wet pick resistance, which canbe determined by an IGT (Institut voor Graphisce Technieck) device. Thisproperty is very important in the field of coated paper as it makes itpossible to obtain a good printing quality, in particular in the case ofoffset printing. This is because, during the printing process,successive applications of different coloured greasy inks are carriedout on the paper. In order to have well defined printing, inkingrollers, which act as mould, are covered with regions of greasy ink andwith regions of wetting water, which is a an aqueous-based composition,these wetting water regions preventing the greasy ink from printing thepaper in the regions where it must not be printed. The presence of wateron the roller thus makes it possible to delimit the regions to beprinted. This water will thus moisten the paper during the printingprocess. The ink is then applied to a slightly wet paper and it isnecessary for the layer to be resistant to this moisture, for qualityprinting.

In order to produce the coated paper, one or more fine layers of coatingcompositions is/are thus applied and is/are dried on the paper. Thiscomposition is applied by continuous coating on a paper, generally byfilm press or blade coating techniques.

The thickness of the layer can be regulated by adjusting the pressureand the speed of forward progression of the paper.

This coated paper or board can subsequently be used in printingprocesses, for example, of offset, photogravure, rotogravure,lithogravure, inkjet or flexography type.

In the coating colour, the binders have the role of causing the pigmentsto stick to one another in the layer obtained and of causing the layerto adhere to the paper. This layer is obtained after drying said colourduring the process of coating the paper.

The amounts of pigments in the colour are generally very high (generallygreater than 50% of the solids content of the colour). Furthermore, thiscolour generally comprises relatively limited amounts of water,generally less than 60%, even less than 50%, by total weight of thecoating colour. This is because, as the layer is obtained by drying thecolour applied during the coating process, it is advantageous for theamount of water in the colour to be low in order to be able to rapidlyobtain the coated paper by evaporation of the water and to use littleenergy in removing this water.

At the same time, it is also necessary for the viscosity of the coatingcolour not to be excessively high, despite the small amount of waterincluded in the colour. This makes it possible to easily apply thecolour: if the latter is not fluid enough, it is necessary to slow downthe coating process in order to obtain a layer having the desiredthickness.

The binders generally used in the colours are styrene polymers, forexample of styrene-butadiene or styrene-acrylate type, which are used inthe form of dispersions in water, otherwise known as “latexes”. Theselatexes are fluid, which makes it possible to manufacture colours havinga rheological behaviour particularly suitable for the process of papercoating.

Furthermore, the properties of the paper obtained are excellent, whichis explained by an excellent adhesion of the fillers to one another andby very good bonding of the layer to the paper.

The printability and the strength of the coated paper obtained arefurthermore excellent.

However, these dispersions can be relatively unstable depending on thestorage conditions. This storage cannot be carried out in dry form,which involves, due to the presence of water, additional transportationcosts. Furthermore, they are produced from non-renewable petroleumresources. These dispersions can thus be relatively expensive. Finally,the coating colours comprising solely conventional styrene dispersionsas binder exhibit a poor water retention, that is to say that water hasto be continually added to the colour during the coating process inorder for this colour to retain its initial viscosity and for themachineability of the colour to be maintained during the process.

Thus, it has been proposed for some years to replace these dispersions,in full or in part, with products of renewable origins. These productscan be modified starches, which have to be at least partially dissolvedin the colour in order to be able to contribute the role of binder.

The modified starches can be prepared by various types of conversions,for example chemically or enzymatically.

Schematically, it is generally considered that, in order to increase thesolubility of a starch, it is necessary to reduce the molecular weightthereof.

Thus, if these modified starches exhibit a high molecular weight, thelatter is combined with a low solubility in water. Mention may be made,by way of example, of the modified starch Stabilys® A023, which is adextrin sold by the Applicant Company, which exhibits a weight-averagemolecular weight of approximately 300 000 Da and a solubility in waterof approximately 5%. In contrast, these modified starches can also behighly soluble derivatives exhibiting a low molecular weight. This isthe case with the dextrin Stabilys® A046, also sold by the ApplicantCompany, which exhibits a weight-average molecular weight ofapproximately 25 000 Da and a solubility in water of approximately 90%.

Mention may be made, as an example of a coating colour comprising amodified starch, of the document WO 2005/003456 A1 owned by theApplicant, which describes an intermediate composition of a specificmodified leguminous starch which can be used in coating colours.

However, these intermediate compositions exhibit the disadvantage ofhaving a viscosity which increases strongly with the amount of starch.Although it is possible to use them, this presents the problem ofrelatively high viscosity of the colour finally obtained. The result ofthis is that the process has to be greatly slowed down, in comparisonwith a process using styrene polymer dispersions, in order to be able tocarry out the coating of the paper.

The document WO 2008/074957 describes the use of modified starch in themanufacture of coating colours exhibiting a high molecular weight and aspecific sol/gel transition temperature. These modified starches areslightly soluble at room temperature. They are subjected to a step ofcooking in a Jet-Cooker in aqueous solution before being used in themanufacture of the coating colour. The result of this is an aqueouscomposition having a relatively high solids content, which is used tomanufacture the coating composition, this composition being more fluidthan those prepared from conventional modified starches. The propertiesof the coated paper obtained are quite satisfactory.

However, even if the problems of high viscosity and of dry weight of thecoating composition are partially solved, it still remains necessary toimprove them.

The document EP 1 964 969 A1 describes, for its part, a coating colourwhich comprises a starch of precise molecular weight and precisesolubilities which exhibits a granular structure. This starch makes itpossible to reduce the cost of the coating compositions, in comparisonwith the compositions comprising other cold-soluble starches, whilemaking it possible to increase the dry weight of the coatingcompositions.

However, this coating colour is unsatisfactory, particularly in terms ofwet pick resistance.

It is to the credit of the Applicant to have been able to manufacturenovel coating colours from novel modified starches, which exhibitproperties improved with respect to those of the prior art.

SUMMARY OF THE INVENTION

A subject-matter of the invention is thus a coating colour comprisingwater, at least one pigment and at least one modified starch, saidmodified starch exhibiting:

-   -   a weight-average molecular weight ranging from 250 000 to 2 000        000 Da;    -   a solubility, measured according to a test A, ranging from 50 to        85%.

The modified starches included in the coating colour of the inventionexhibit the advantage of combining a relatively high solubility in waterand a relatively high molecular weight. Thanks to the combination ofthese a priori contradictory properties, these colours canadvantageously be used in the coating field. This is because, inaddition to fulfilling the requirements already presented, the coloursaccording to the invention have the advantage of making it possible toobtain a coating process of high machineability, in comparison with theprocesses using the slips already known. The coated paper obtainedexhibits the advantage of exhibiting good properties and in particularan excellent wet pick resistance.

DETAILED DESCRIPTION

The modified starch useful in the invention thus exhibits specificcharacteristics of molecular weight and aqueous solubility.

As regards the molecular weight (W) of the modified starch, it isexpressed in Da and can be determined by a person skilled in the artusing size-exclusion chromatography of HPSEC-MALLS (High PerformanceSize Exclusion Chromatography coupled on-line with Multiple Angle LaserLight Scattering) type.

This weight can be measured by size-exclusion chromatography accordingto the following protocol:

-   -   preparation of a sample by dissolution of the modified starch,        by heating at 100° C. for 30 min in a dilution solvent composed        of a DMSO/NaNO₃ mixture (0.1M NaNO₃ in DMSO), it being possible        for said sample to exhibit a concentration ranging from 2 to 10        mg of starch per ml of dilution solvent;    -   use of a high performance liquid chromatography (HPLC) device        equipped with a pump, operating in isocratic mode, which causes        an elution solvent to circulate at 0.3 ml/min, with a        refractometer, with a laser multiple angle light scattering        detector having 18 angles which is heated at 35° C., for example        a DAWN DSP detector from Wyatt, and with an oven for        thermostatic control of the columns heated at 35° C., for        example equipped with polyhydroxymethacrylate columns of Suprema        type for which the elution solvent is, for example, a 0.1M        aqueous sodium nitrate solution comprising 0.02% by weight of        sodium azide;    -   injection into the device of approximately 100 μl of the sample.

The weight-average and number-average molecular weights can bedetermined from the spectra obtained, for example by reprocessing thespectra as exponential 1^(st) order, using analytical software of Astrav.4 type.

For its part, the solubility is measured according to the test A, whichconsists of the following method:

-   -   taking a weight of test sample (w test sample) of approximately        5.0 g of modified starch, this weight being expressed as dry        weight;    -   dispersing in 200 ml of water at 22° C. in an Erlenmeyer flask,        which is subsequently placed at 22° C. in a water bath for 4        hours under magnetic stirring with stirring for 5 minutes every        30 minutes;    -   filtering the contents of the flask through a filter with a        porosity of 8 μm, for example of Whattamn 2 V type;    -   pipetting 50 ml of filtrate and introducing this amount into a        dry and tared crystallizing dish;    -   evaporating the water from the crystallizing dish by heating at        60° C. for 45 minutes and then at 130° C. for 90 minutes;    -   after cooling in a desiccator, weighing the weight of starch        obtained after drying (w solids extract).

The solubility is calculated in the following way:Solubility=[w solids extract×200×100]/[50×w test sample]

The dry weight of the test sample is calculated according to StandardISO 1666:1996.

The modified starch useful in the invention advantageously exhibits apolydispersity index of greater than 5, preferably of greater than 10and more preferably still of greater than 15.

The modified starch according to the invention can exhibit anumber-average molecular weight of greater than 20 000 Da, preferably ofgreater than 25 000 Da and more preferably still of greater than 30 000Da.

Preferably, the solubility of the modified starch ranges from 55 to 65%.

Preferably, the modified starch exhibits a weight-average molecularweight ranging from 380 000 Da to 1 400 000 Da.

The modified starch useful in the invention can exhibit a mass fractionof greater than 5% of starch molecules with a molecular weight ofgreater than 1 000 000 Da, preferably a fraction of between 5 and 25%.

Preferably, the modified starch comprises, with respect to the totalweight of modified starch:

-   -   a mass fraction of between 5 and 25% of starch molecules with a        molecular weight of greater than 1 000 000 Da,    -   a mass fraction of between 25 and 50% of starch molecules with a        molecular weight of greater than 100 000 Da and less than or        equal to 1 000 000 Da,    -   a mass fraction of between 30 and 50% of starch molecules with a        molecular weight of greater than 10 000 Da and less than or        equal to 100 000 Da,    -   a mass fraction of less than 20% of starch molecules with a        molecular weight of less than or equal to 10 000 Da.

The modified starch useful in the invention can be a dextrin, that is tosay that it can be obtained by a dextrinization step.

Numerous processes for modifying starch and in particular dextrinizationprocesses have been developed, these processes using the action of heat,in a dry or non-dry medium, in the presence or absence of chemicalagent. For the most part, whether batchwise or continuous, theseprocesses involve transformation temperatures of greater than 100° C.and the optional presence of an acid, of an alkaline agent and/or of anoxidizing agent.

Dextrins are generally classified into three categories:

-   -   white dextrins, generally obtained by transformation of the        starch at temperatures often of between 120 and 170° C., in the        presence of chemical agent(s), in particular of acid, in        relatively high amounts,    -   yellow dextrins, often manufactured by transformation of the        starch at higher temperatures, generally between 170 and 230°        C., in the presence of chemical agent(s), in particular of acid,    -   dextrins known as “British Gum”, obtained by the action of heat        alone, at high temperature, often of greater than 230° C.

The dextrinization processes generate different reactions. Theimportance of each of them varies with the essential parameters, such asthe nature and the amount of chemical agent, the water content duringreaction, the temperature profile and, to a lesser extent, the reactiontime.

A person skilled in the art generally considers that various mechanismscan occur during the reaction for the dextrinization of starch.

The hydrolysis reaction is significant at the start of thetransformation, this being the case from 50° C. Related to the presenceof acid and of a still-sufficient quantity of water, it reduces themolecular weight.

The condensation or reversion reaction forms an α(1.6) bond from aprimary alcohol of one chain and the reducing end of another chain. Itis promoted by temperatures of less than or close to 150° C.

The “transglucosidation” reaction, forming an α(1.6) bond while cleavingan α(1.4) bond without releasing water, is predominant at temperaturesof greater than 150° C. As it makes it possible to obtain more branchedmolecules, it is essential to the expression of the properties of thedextrins, in particular yellow dextrins.

Other reactions also take place, such as internal “anhydridization”,between carbons 1 and 6, or the recombination resulting from thereaction between a reducing end and a C₂, C₃ or C₄ hydroxyl group.

The relative importance of these phenomena confers specific propertieson the dextrins.

In the transformation of white dextrins, the predominant mechanism ishydrolysis. The result is that these white dextrins exhibit highmolecular weight with low solubility in water, or the contrary. In thecase of yellow dextrins, hydrolysis is important and all the mechanismsdescribed above have an equal importance, which results in products oflow molecular weight, generally rebranched, exhibiting a high solubilityin water.

The properties of the dextrins thus result from the competition of thevarious complex reaction mechanisms described above. The Applicant hassucceeded, thanks to a process using specific operating conditions, inmanufacturing, from starch, a modified starch exhibiting propertieswhich are a priori contradictory, i.e. exhibiting a high solubility inwater but also a high molecular weight. Without being committed to anyone theory, the Applicant explains that this modified starch is obtainedby the choice of conditions which make it possible to promote, morepreferably than usual, rebranching or recombination reactions, whichincrease the molecular weight, while limiting cleavage reactions, whichreduce the molecular weight (hydrolysis reactions). The relativeproportions of these reactions and mechanisms, occurring under veryspecific conditions, result in macromolecular structures of high weightexhibiting a high solubility in water.

The Applicant has succeeded in obtaining this modified starch bycarrying out the reaction on a starch of granular form using specificconditions.

In the present case, in order to obtain the modified starch useful inthe invention, which is a dextrin, the Applicant has developed a processfor modifying starch, which comprises:

-   -   a step of introducing at least one granular starch into a        reactor;    -   a step of introducing, into said reactor, at least one acidic        agent chosen from among strong acids;    -   a step of modifying the starch in said reactor;    -   a step of recovering the starch modified during the preceding        step;        in which at least part of the modifying step is carried out        with:    -   a moisture content of the starch of between 1 and 3%;    -   an amount of acid introduced of between 0.003 and 0.020 mol of        acid per kg of dry starch, preferably between 0.006 and 0.015        mol and more preferably between 0.008 and 0.012 mol;    -   and a temperature of the reactor of between 160 and 215° C.,        preferably between 170 and 210° C.

At the end of the process, a modified starch is recovered which exhibitsthe molecular weight and the solubility defined above. The structure ofthe modified starch obtained is generally granular.

The duration of the modifying step, subsequently referred to as “contacttime”, is advantageously between 1 and 10 minutes, preferably between 3and 7 minutes.

The period of time during which the modifying takes place at a moisturecontent of between 1 and 3% is at least 10 seconds, preferably at least30 seconds and more preferably still at least 1 minute.

Throughout the step of modifying the starch, it is preferable for thecontent of the starch never to be less than 1%.

Various processes for modifying starch carried out in the dry phase,under hot conditions and in the presence of acid, have already beendescribed in the prior art.

For example, according to the document EP 710 670 A1, use is made of aprocess in which a maize starch is introduced with an amount by weightof hydrochloric acid of 0.63% with respect to the weight of the starch,i.e. a molar amount of approximately 0.14 mol per kg of starch. Thetemperature during the process is at most 120° C. and the moisturecontent at the end of the process is approximately 11%. A starch with alow weight-average molecular weight, far below 250 000 Da, is obtained.

Mention may also be made of U.S. Pat. No. 6,191,116 B1, which describesa completely soluble dextrin manufactured by a process in which ananhydrous starch is modified at a temperature ranging from 170 to 210°C. with an amount by weight of acid, preferably hydrochloric acid, of0.25% with respect to the weight of the starch, that is to say a molaramount of approximately 0.07 mol per kg of starch.

These processes of the prior art have not made it possible to obtain themodified starch of the invention.

The starch introduced during the process of the invention is in agranular form.

Within the meaning of the present invention, “granular starch” isunderstood to mean a native starch or a modified starch which hasretained a semicrystalline structure similar to that demonstrated in thestarch grains naturally present in the storage organs and tissues ofhigher plants, in particular in cereal seeds, legume seeds, potato orcassava tubers, roots, bulbs, stems and fruits.

This semicrystalline state is essentially due to the amylopectinmacromolecules, one of the two main constituents of starch. In thenative state, starch grains exhibit a degree of crystallinity whichvaries from 15 to 45% and which depends essentially on the botanicalorigin of the starch and on the possible treatment to which it has beensubjected. Granular starch, placed under polarized light, exhibitsmicroscopically a characteristic black cross, referred to as “Maltesecross”, typical of the crystalline granular state. For a more detaileddescription of granular starch, reference may be made in particular tochapter II, entitled “Structure et morphologie du grain d'amidon”[Structure and morphology of the starch grain] by S. Perez in the work“Initiation á la chimie et á la physico-chimie macromoleculaires”[Introduction to macromolecular chemistry and physical chemistry], firstedition, 2000, Volume 13, pages 41 to 86, Groupe Francais d'Etudes etd'Applications des Polymeres.

A starch granule comprises amylose, amylopectin and water. The moisturecontent of granular starch, under standard conditions, varies accordingto the botanical nature of the starch. This is because granular starchcomprises an intrinsic amount of water and the moisture content in thisstarch is generally between 10 and 20%. By way of examples, a maizestarch exhibits a moisture content under standard conditions ofapproximately 13%, while a potato flour exhibits a moisture content ofapproximately 18%. Thus, when 1% of water is added to a maize starch, astarch exhibiting a moisture content of approximately 14% is obtained.

According to the process for modifying the starch useful in theinvention, at least part of the step of modifying the starch is carriedout with a low moisture content in the starch, for example of between 1and 3%. It is specified that the moisture content of the starch consistsof the water/dry starch ratio by weight and that it can be measuredaccording to Standard ISO 1666:1996.

The modifying step is carried out using an acidic agent chosen fromstrong acids. The step of introducing the acidic agent of use in themodifying step can be carried out using an aqueous acidic solution. Theamount of acidic solution thus introduced into the reactor is low,advantageously between 0 and 10% of the weight of dry starch, preferablybetween 0.5 and 4%.

A person skilled in the art understands, by strong acid, an acidexhibiting a pKa<−1.7. This acidic agent can be chosen in particularfrom methanesulphonic acid, sulphuric acid, hydrochloric acid or nitricacid, or one of their mixtures. Advantageously, the acidic agent ismethanesulphonic acid, sulphuric acid, hydrochloric acid, or a mixtureof at least two of these acids, preferably sulphuric acid. In theprocess according to the invention, the amount of acidic agent isbetween 0.003 and 0.020 mol of acid per 1 kg of dry starch, which isequivalent, in the case of hydrochloric acid, to amounts by weight ofacid, with respect to the starch, within amounts ranging approximatelyfrom 0.01 to 0.08% (dry/dry) and, in the case of sulphuric acid ormethanesulphonic acid, to amounts by weight of acid, with respect to thestarch, within amounts ranging approximately from 0.03 to 0.2%(dry/dry).

The step of introducing the acidic agent into the reactor can be carriedout simultaneously with the step of introducing the starch, for exampleby introducing a mixture of starch and acid. Said mixture can beproduced before they are introduced into the reactor by simplemechanical mixing. An example of a mixer which can be used to this endis the CB 20 sold by Lödige®. Generally, the acid is introduced into thereactor in the form of an aqueous acidic solution.

The total amount of water of the starch is composed of the amount ofwater intrinsically present in the starch and of the amount of wateroptionally added to this starch in the reactor. The moisture content ofthe starch during the process is the ratio by weight of the total amountof water to the sum of the total amount of water and anhydrous starch.It is advantageous for the moisture content of the starch at thebeginning of the reaction not to exceed 35%, for it to be preferablyless than 25%, more preferably still less than 18% and even less than16%.

The process for modifying the starch useful in the invention can becarried out in any type of reactor, for example a batch reactor or acontinuous tubular reactor.

Preferably, the modifying step is carried out in the cylindrical linearturboreactor described above which is well known to a person skilled inthe art. Such a type of reactor is sold, for example, by Vomm® and isdescribed in detail in the document EP 710 670 A1.

The turboreactor comprises a rotating shaft provided with blades, thefunction of which is to continuously convey the starch. The starch isintroduced at the head of the turboreactor and is then conveyed throughthe reactor using the blades.

The contact time can be chosen by modifying the rotational speed of theshaft. It is also possible to increase these times by carrying out atleast two passes of the starch through the turboreactor.

In order to carry out the modifying step, it is necessary to remove aportion of the water from the starch since, as explained above, understandard conditions, the moisture content of granular starch is between10 and 20%. It is thus possible to carry out a step of preliminarydrying of the starch before carrying out the modifying step itself.

Use may be made of any dryer known to a person skilled in the art inorder to carry out this drying step, for example an oven, a fluidizedbed or a dryer of “turbodryer” type. A turbodryer is produced in aturboreactor by bringing about the circulation in said turboreactor of astream of air, referred to in the continuation of the description as“drying stream”. This turbodryer is also described in detail in theabovementioned document EP 710 670 A1.

The air of the drying stream preferably has a temperature of between 50and 250° C., preferably between 150 and 220° C. The air flow rate of thestream can also be varied. The drying stream can be chosen to circulatein the direction of conveyance of the starch or in the oppositedirection. Preferably, the air stream travels in the direction ofconveyance. The drying stream can have a flow rate of between 10 and1000 Sm³/h, for example from 30 to 500 Sm³/h. The conditions of thedrying stream set out above are, however, chosen in order for themoisture content in the starch to be between 1 and 3% during at leastpart of the modifying step.

Advantageously, the reactor is a turboreactor and the drying and themodifying of the starch are carried out simultaneously by use of adrying stream.

The step of introducing the acidic agent into the turboreactor can becarried out simultaneously with the step of introducing the starch, forexample by introducing a mixture of starch and acid. Said mixing can becarried out before their introduction into the turboreactor by simplemechanical mixing. An example of a mechanical mixer which can be used tothis end is the CB 20 sold by Lödige®.

Thus, very particularly, the process for modifying starch comprises:

-   -   a step of mixing at least one granular starch with an aqueous        solution comprising an acidic agent chosen from strong acids;    -   a step of introducing the mixture formed during the preceding        step into a turboreactor in the presence of a drying stream;    -   a step of modifying the starch in said turboreactor;    -   a step of recovering the modified starch obtained during the        preceding step;        in which at least part of the modifying step is carried out        with:    -   a moisture content of the starch of between 1 and 3%;    -   an amount of acid introduced of between 0.003 and 0.020 mol of        acid per kg of dry starch, preferably between 0.006 and 0.015        mol, indeed even between 0.008 and 0.012 mol;    -   and a temperature of the reactor of between 160 and 215° C.,        preferably between 170 and 210° C.;        the duration of the modifying step advantageously being between        1 and 10 minutes, preferably between 3 and 7 minutes, and the        moisture content of the starch during the modifying step being        greater than 1%.

At the end of the modifying process, the modified starch can beneutralized using a base, for example using sodium hydroxide or sodiumcarbonate.

Various processes for obtaining the modified starch useful in theinvention appear in the examples.

The modified starch useful in the invention exhibits the advantage ofbeing able to easily form, by simple mixing in water and in particularin water at room temperature, a solution capable of adhesively bondingto numerous supports, which will subsequently be referred to as aqueousbinder.

The aqueous binder obtained from the modified starch of the inventionexhibits the advantage of having a relatively low viscosity, even whenthe amount of starch in the binder is high.

Without being committed to any one theory, the Applicant explains theseproperties by the specific solubility and the molecular weight of themodified starch in accordance with the invention.

The aqueous binder can be an aqueous composition comprising, by weight,from 0.01 to 90% of modified starch according to the invention, forexample from 1 to 25%.

The Applicant Company has found that these particularly advantageousproperties make it possible to use this modified starch to manufacture acoating colour exhibiting excellent properties.

In addition to the modified starch, the coating colour additionallycomprises water and one or more pigments.

The coating colour according to the invention can comprise, in additionto the water, optionally small amounts of at least one other solvent,such as an alcohol, but it is preferable for the solvent of the colourto be composed of water.

The coating colour advantageously exhibits a solids content of between40 and 85%, preferably between 50 and 75% and more preferably stillbetween 55 and 73%.

Examples of appropriate pigments comprise clays, such as kaolin, butalso calcined clays, hydrated aluminium silicates, bentonite, calciumcarbonate, calcium sulphate or gypsum, silicas, in particularprecipitated silicas, titanium dioxide, alumina, aluminium trihydrate,plastic pigments, such as those made of polystyrene, satin white, talc,barium sulphate, zinc oxide and mixtures of two or more of them.

The appropriate pigment or pigments can be easily selected by a personskilled in the art according to the type of coating colour to beobtained. To prepare the coating colour according to the invention, thepigment can be added in the form of a dispersion of the pigment inwater.

The coating colour according to the invention can also comprise one ormore additional polymers having the role of binder, other than themodified starch of the invention. This additional polymer can be apolymer chosen from styrene polymers, for example a styrene/butadiene,styrene/acrylate or styrene/butadiene/acrylate copolymer, polymers of(meth)acrylic type or of (meth)acrylate type, saponified ornonsaponified polyvinyl acetates, starches other than the modifiedstarch according to the invention, and protein binders, such as casein,gelatin or soya proteins. Preference is given, as additional polymer, toa polymer chosen from styrene polymers.

These additional polymers can advantageously be introduced into thecomposition via a dispersion in water.

The coating colour according to the invention can comprise, per 100parts of pigments, from 1 to 100 parts of modified starch and from 0 to20 parts of additional polymer, the sum of the parts of modified starchand additional polymer ranging from 3 to 100 parts.

Advantageously, the coating colour comprises, per 100 parts of pigments,from 1 to 25 parts of modified starch and from 0 to 10 parts ofadditional polymer, the sum of the parts of modified starch andadditional polymer ranging from 4 to 30. Preferably, the coating colourcomprises, per 100 parts of pigments, from 2 to 15 parts of modifiedstarch and from 2 to 7 part of additional polymer, the sum of the partsof modified starch and additional polymer ranging from 6 to 25 parts.

The coating colour generally comprises a pH regulator, for example abase, such as sodium hydroxide, or an acid, such as hydrochloric acid.

The pH of the colour, which generally ranges from 8 to 10.5 andpreferably from 8.5 to 10, is adjusted by adding selected amounts of pHregulator.

The coating colour can also comprise additives conventionally used incoating colours.

Mention may be made, as additives, of thickeners. Examples ofappropriate thickeners comprise synthetic thickeners or thickeners ofnatural origin, such as cellulose ethers, for examplecarboxymethylcellulose (CMC), hydroxyethylcellulose,hydroxypropylcellulose, ethylhydroxyethylcellulose or methylcellulose,alginates, such as sodium alginate, xanthan gum, carrageenans andgalactomannans, such as guar gum.

Other additives which can be used are surfactants, for example cationicsurfactants, anionic, nonionic or amphoteric surfactants, andfluorinated surfactants.

Hardeners can also be used, such as, for example, active halogencompounds, vinyl sulphone compounds or epoxy compounds.

Dispersing agents, such as polyphosphates or polycarboxylates, can alsobe used.

Mention may be made, as examples of additives which can also be includedin the coating colour of the invention, of agents which improve thefluidity, lubricants, antifoaming agents, optical brighteners,preservatives, such as, for example, benzisothiazolone and isothiazolonecompounds, or biocidal agents, such as, for example, sodium metaborate,thiocyanate and benzoate.

Other additives which may be mentioned are also yellowing inhibitors,such as sodium sulphonate derivatives, ultraviolet absorbents orantioxidants.

Agents for resistance to water, which can be chosen from ketone resins,anionic latexes or glyoxal, and/or wet or dry strength agents, such asglyoxal-based resins, polyethylene oxides, or melamine orurea-formaldehyde resins, can also be used.

Crosslinking agents, additives for retaining the gloss of the ink oradditives for resistance to greases and oil can also be included in thecoating colour of the invention.

The person skilled in the art can select the additives in order toobtain the coating colour and the paper having the desired properties.The coating colour can comprise, per 100 parts of pigments, from 0 to 5parts of additives.

The coating colour can be prepared by simple mixing of the variousconstituents of the composition: the invention thus relates to a processfor manufacturing a coating colour comprising a step of mixing pigments,modified starch, water and optional additional polymer, in which themodified starch is introduced into the colour without having beensubjected to a precooking step.

The invention thus also relates to a process for preparing a colourcomprising:

-   -   a step of mixing the water, the pigment, the modified starch        according to the invention and the additional polymer, indeed        even the optional additives;    -   a step of recovering the coating colour;        in which the process is carried out at a temperature ranging        from 5 to 50° C., preferably from 15 to 40° C.

Preferably, the pigment and the optional additional polymer areintroduced in the form of dispersions in water, which makes it possibleto facilitate the preparation of the colour. In particular, a coatingcolour can be manufactured as follows: a dispersion of pigments, water,optionally a dispersion of additional polymer and/or additives are mixedat a temperature ranging from 5 to 50° C., preferably from 15 to 40° C.and preferably at room temperature, and then the modified starch of theinvention is added. After mixing, a coating colour is obtained which iscapable of being used in a coating process.

A person skilled in the art can select the amounts of each of theconstituents in order to have the viscosity which will make it possibleto apply the composition to the paper in a satisfactory way during thecoating process. For example, the coating colour according to theinvention exhibits a Brookfield viscosity ranging from 500 to 2000mPa·s, preferably between 600 and 1200 mPa·s, preferably between 600 and900 mPa·s, and/or a Hercules viscosity ranging from 40 to 80 mPa·s. TheBrookfield viscosity can be measured using a viscometer having theBrookfield trade name, the rotational speed of the spindle of which is100 revolutions/minute. The Hercules viscosity can be measured using aviscometer having the Hercules trade name equipped with an “FF” bob, therotational speed of which is 6000 revolutions/minute. The viscosity ofthe colour increases mainly with the solids content, the amount ofpigments and the amounts of starch, additional polymer and thickener. Asthe coating colour according to the invention exhibits the advantage ofbeing able to exhibit a lower viscosity than some starch-based coatingcolours of the prior art, this makes it possible to be able to usegreater amounts of modified starch in the colour for an equivalentviscosity.

Another subject-matter of the invention is a process for coating paper,comprising at least one step of coating a paper with the coating colouraccording to the invention, preferably from 1 to 5 steps and morepreferably still from 2 to 3 coating steps.

The coating process according to the invention exhibits the advantage ofbeing able to use greater coating rates than some starch-based coatingcolours of the prior art, which makes it possible to increase theproductive output of the paper coating processes.

According to the invention, the term paper encompasses any type of paperor board capable of being subjected to a coating treatment.

The coating of the paper can be carried out in line in the papermakingmachine or on a separate coating machine.

Depending on the quality of the paper or board desired and its finaluse, it can be coated on just one or both of its faces. Each face of thepaper can be coated just once or several times, on one or both of itsfaces, provided that at least one of the layers is produced from thecoating colour according to the invention. By way of example, a coatedpaper of high quality generally comprises three layers per side.

To produce a layer, first of all a covering step is carried out,followed by a drying step.

The paper-covering step according to the invention can be carried out byany covering process known to a person skilled in the art. It can becarried out, for example, by air-knife coating, pencil coating, smoothrod coating, threaded rod coating, spray coating, knife coating, curtaincoating, size press coating, film press coating, brush coating, castcoating, gravure coating, jet applicator coating, short dwell coating,slide hopper coating, flexographic coating and reverse roll coating.

After the covering step, the layer is obtained by a drying step. Thisdrying step can be carried out by drying with air, by convection, bycontact or by radiation, for example by infrared radiation.

Optionally, the coating process can comprise a calendering step, whichmakes it possible to increase the gloss and the smoothness of the coatedpaper.

The calendering step is generally carried out by passing the covered anddried paper through a calender or between rollers which are generallycovered with elastomers. For better results, the calendering can becarried out with heated rollers.

Another subject-matter of the invention is a coated paper obtained fromthe coating process of the invention.

The paper of the invention exhibits better properties, in particular awet strength which is superior to those of the coated papers obtainedfrom starch-based coating colours of the prior art.

Each layer obtained by the process of the invention can comprise anamount of material ranging from 1 to 30 g/m², for example from 4 to 25g/m² and preferably from 6 to 20 g/m².

The coated paper can comprise from 1 to 5 coating layers, preferablyfrom 2 to 3 coating layers, at least one of which is obtained from thecoating colour according to the invention.

The paper according to the invention exhibits an improved wet pickresistance with respect to that of the coated papers of the prior art,the layers of which are based on starch, it being possible for thisresistance to be, according to the method described in the examples,greater than 60 m/s, advantageously greater than 75 m/s and preferablygreater than 85 m/s.

Due to its superior properties, the coated paper according to theinvention can advantageously be used in a printing process, such as, forexample, printing of offset, heliogravure, rotogravure, lithogravure,ink jet or flexography type, preferably of offset type.

The present invention will now be illustrated in detail via exampleswhich do not in any way limit the invention claimed.

Examples of the Manufacture of Starch

Use is made, as starting materials in the preparation of the modifiedstarch useful in the invention, of a native maize starch, acid andwater. The starch was modified using hydrochloric acid (HCl), sulphuricacid (H₂SO₄) or methanesulphonic acid (CH₄O₃S).

This modified starch is obtained by a dextrinization reaction accordingto the following protocol.

A premix of these three starting materials is prepared on a mixer ofLödige CB20 type by introducing maize starch, the relative moisturecontent of which is approximately 13%, one of the abovementioned acidsand water. The moisture content of the premix thus formed and the numberof moles of acid per kilogram of native starch are shown in Tables 1 and2, which summarize the various tests carried out. They are respectivelygiven in the columns entitled “% water” and “n”.

The premix is subsequently introduced continuously at a flow rate byweight of 50 kg/h and conveyed through a cylindrical linear turbodryersold by the manufacturer Vomm, exhibiting a rotating shaft provided withblades which rotates at a speed of 1000 revolutions/min. The temperatureof the reactor is shown for the various tests in Tables 1 and 2. Anairflow, having a temperature identical to that of the reactor, isintroduced into the turboreactor and travels through this reactor in thedirection of conveyance of the starch with a flow rate of 50 Sm³/h,which makes it possible to dry the starch during the reaction. Themoisture content of the starch (Mc) is also analyzed according toStandard ISO 1666:1996 at the end of the reaction.

Tests 1 to 14 are carried out by performing one pass through theturbodryer, regulated at the temperature T, and the modified starch isrecovered after said pass. The duration of the step of modifying thestarch is approximately 4 minutes. These tests 1 to 14 are broughttogether in Table 1.

Tests 15 to 22 are carried out by performing a first pass of the starchthrough the turbodryer at the temperature T1 and then a second pass atthe temperature T2. The duration of the process of modifying the starchis approximately 8 minutes. These tests 15 to 22 are brought together inTable 2.

For the various tests, the solubility of the modified starch and alsoits weight-average molecular weight, its number-average molecular weightand the polydispersity index (M_(w)/M_(n)) are given in Tables 1 and 2.

For reasons of readability, the letter “I” appears in the test typecolumn of the table for the modified starches useful in the invention,whereas “CP” is shown in the same column when different modifiedstarches are concerned.

The mass fractions of the starch molecules constituting the modifiedstarches are also measured for some of these starches.

These mass fractions were analyzed for the modified starches obtained inExamples 2, 14, 15, 16, 21 and 22, and also for the product C* ICOAT07520, sold by Cargill, which exhibits a weight-average molecular weightof 230 000 Da and a solubility of 67%.

The various tests show that the process of the invention, which usesspecific conditions during the modifying step, makes it possible toobtain modified starches combining properties hitherto incompatible: ahigh solubility and a very high weight-average molecular weight.

TABLE 1 Manufacture of modified starches with one pass through theturbodryer Conditions of Conditions for preparing the modifying thestarch mixture process Characteristics of the modified starch obtainedTest n % T Mc Solubility type Ex. Acid (mol/kg) water (° C.) (%) (%)M_(w) (Da) M_(n) (Da) M_(w)/M_(n) CP 1 HCl 0.011 22.5 140 7.4 2.8 422112 Not Not calculated calculated I 2 HCl 0.011 14 180 2.6 70.3 660 31230 023 22.0 I 3 HCl 0.011 14 200 1.8 81.1 480 214 18 131 26.5 CP 4 HCl0.011 14 220 1.4 91.3 219 851 13 005 16.9 I 5 HCl 0.011 16 200 1.7 80.5440 245 20 120 21.9 CP 6 HCl 0.009 22.5 140 5.3 65.5 169 250 25 830 6.6I 7 HCl 0.009 22.5 200 2.1 56.5 664 321 27 412 24.2 I 8 HCl 0.009 16 1802.6 57 1 204 325 54 398 22.1 I 9 HCl 0.009 16 200 1.9 71 900 450 35 03525.7 CP 10 HCl 0.009 16 220 1.8 86 480 217 21 872 22.0 I 11 HCl 0.009 14180 3 51 1 200 138 48 231 24.9 I 12 HCl 0.009 14 200 2.3 70 702 543 27432 25.6 CP 13 HCl 0.009 14 220 1.9 87.5 289 342 16 350 17.7 I 14 H₂SO₄0.011 22.5 200 2.4 60.4 1 204 069 64 397 18.7

TABLE 2 Manufacture of modified starches with two passes through theturbodryer Conditions for Conditions preparing the starch for modifyingmixture the starch Characteristics of the modified starch obtained Testn % T1 T2 Mc Solubility type Ex. Acid (mol/kg) water (° C.) (° C.) (%)(%) M_(w) (Da) M_(n) (Da) M_(w)/M_(n) I 15 HCl 0.011 22.5 200 140 1.862.8 398 326 Not Not calculated calculated CP 16 HCl 0.022 16 200 1401.7 90 25 523 5987 4.3 CP 17 HCl 0.022 16 170 180 1.4 98 24 123 Not Notcalculated calculated CP 18 HCl 0.011 22.5 140 140 3.7 8.3 1 100 421 NotNot calculated calculated I 19 HCl 0.011 22.5 140 200 2.8 61.9 422 47820 365 20.7 I 20 HCl 0.011 22.5 200 200 1.6 80.1 326 185 18 238 17.9 CP21 HCl 0.011 16 140 140 3 65.3 96 580 19 857 9.9 I 22 CH₄O₃S 0.011 22.5200 200 2 62.5 983 247 46 237 21.3

TABLE 3 Mass fractions of starch molecules in the modified starch of theinvention Fractions Ex. 2 Ex. 14 Ex. 15 Ex. 16 Ex. 21 Ex. 22 C* 07520 M≥ 1 000 000 8.5 18.4 6.2 0 14.6 0.9 3.5 10e6 > M ≥ 10e5 36.9 43 31.9 2.140.7 25.1 45.7 10e5 > M ≥ 10e4 44.9 36.7 44.1 70.6 39.8 59.7 45.1 10e4 >M 9.6 1.9 17.8 27.3 4.9 14.4 5.8

Applicative Examples

In order to illustrate the invention, the modified starch will now beused in the manufacture of coating colours in the examples which follow.

Products Used

The following products were used to produce the coating colours:

Styrene-butadiene latex

Pigment 1: Calcium carbonate dispersion

Pigment 2: Kaolin dispersion

Synthetic thickener

Optical brightener

Water

Aqueous sodium hydroxide solution at a concentration of 10 mol·l⁻¹

The modified starches used in the coating colours are as follows:

Modified starches of the invention: starch of Ex. 2, 7, 14, 15 and 22.

Comparative starches: C*ICOAT 07520, starch of Ex. 16 and 21.

Preparation of the Colour

The reference colour (Ref.), free of modified starch, comprises 10 partsof latex. To prepare this colour, the pigment dispersions are stirred ina stainless steel vessel using a Raineri motorized deflocculator. Thelatex is introduced into the vessel after stirring for 10 minutes. After30 minutes, the other constituents, i.e. the thickener, the opticalbrightener, the water and the aqueous sodium hydroxide solution, arealso successively introduced into the vessel, each of these constituentsbeing added 5 minutes after the preceding one. The amount of sodiumhydroxide is adjusted in order to obtain a pH of the colour of 9.

The Brookfield and Hercules viscosities of the colour obtained aresubsequently measured according to the methods described above.

The colours according to the invention (I) and the comparative colours(CP) differ from the reference composition in that 4 parts of latex arereplaced by 7 parts of modified starch.

These colours are prepared in the same way as the reference colour,except that the modified starch is introduced ten minutes afterintroducing the latex and that the other constituents are introducedtwenty minutes after introducing the modified starch.

The compositions of the colours are shown in Table 4 below.

Preparation of the Papers

The coatings were carried out on a base paper, with a grammage of 80g/m², using the Helicoater pilot plant, manufactured by Dixon, with adeposition of colour of 7 to 8 g/m², the speed of the machine beingadjusted to the rheology of the colour. Infrared drying is carried outusing 14 lamps, each of 2000 W, and the drying time is 30 seconds.

The papers were calendered in two passes at 60-65° C. with a pressure of150 daN/cm on a sheet-to-sheet calender from the manufacturer ABK.

Characteristics of the Papers Obtained

The gloss of the papers obtained after calendering was measuredaccording to Standard TAPPI T 480.

The dry pick measurements were determined using an IGT AIC 2-5 deviceequipped with 50 mm rolls covered with a blanket, using an IGT 3803 ink,on paper test specimens with a size of 340×55 mm.

The ink is deposited on the rolls before transfer onto the paper.Transfer is carried out by applying a pressure of 625 N to the roll, thespeed of the sample holder constantly accelerating and reaching a finalspeed of 4 m/s at the end of the sample.

The wet pick is measured according to the same protocol, apart from thedifference that the speed of the sample holder is constant at 0.2 m/sand that transfer of water is carried out before transfer of the ink.This transfer is carried out using a first 38 mm screened roll and byapplying a pressure of 400 N to this roll.

The characteristics of the coating colours produced and of the papersobtained from these colours are described in detail in the table below.

TABLE 4 Applicative results Maximum Test Starch Brookfield Herculesspeed Dry IGT Wet IGT Gloss type No. (mPa · s) (mPa · s) (m/min) (m/s)(m/s) (%) Ref. No starch 500 35 1300 2.18 93 60.7 I  2 640 44 1100 2.3686.7 59.1 I  7 750 51 1100 2.18 90.5 60.8 I 14 600 51 1200 2.18 93.159.9 I 15 700 53 1200 2.36 92.2 60.9 CP 16 730 46 1300 2.36 71.2 58.1 CP21 830 51 1200 2.18 67.5 58.4 I 22 690 53.5 1100 2.36 91 59.8 CP C*ICOAT980 56 1100 2.54 81.9 57.2 07520

The tests have shown that the modified starches according to theinvention 2, 7, 14, 15 and 22 make it possible to obtain coating colourswhich can be applied at a speed close to that of the starch-freereference coating colour. Some of these colours can even be applied atthe same speed as this reference colour. This is also shown by theBrookfield viscosity of the coating colours according to the invention,which, without being as low as the viscosity of the reference colour, islower in comparison with the viscosity of the comparative colours.

The gloss of the coated paper according to the invention is also greaterthan that of the comparative coated papers.

The dry pick resistance is similar, whatever the paper. However, the wetpick resistance is greatly improved since it reaches a level close to,indeed even identical to, that of the reference coated paper. This isparticularly true if the papers obtained from the compositions of theinvention are compared with those obtained from compositions 16 and 21,which are in accordance with the teaching of the document EP 1 964 969A1.

The invention claimed is:
 1. A coating colour comprising water, at leastone pigment and at least one dextrin, wherein the water content of thecoating colour is less than 60% by total weight of the coating colourand the pigment content is greater than 50% by weight of the solidscontent of the coating colour, wherein the at least one dextrin has aweight-average molecular weight ranging from 250,000 to 1,400,000 Da anda solubility, measured according to a test A, ranging from 50 to 85%,and wherein the at least one dextrin is obtained by a process whichcomprises: a step of introducing at least one granular starch into areactor; a step of introducing, into said reactor, at least one acidicagent, said at least one agent being a strong acid; a step of modifyingthe starch in said reactor; a step of recovering the starch modified insaid reactor; wherein at least part of the modifying step is carried outwith: a moisture content of the starch of between 1 and 3%; an amount ofacid introduced of between 0.003 and 0.020 mol of acid per kg of drystarch; and a temperature of the reactor of between 160 and 215° C. 2.The coating colour according to claim 1, wherein the polydispersityindex of the dextrin is greater than
 5. 3. The coating colour accordingto claim 1, wherein the dextrin exhibits a solubility according to thetest A ranging from 55 to 65%.
 4. The coating colour according to claim1, wherein the dextrin exhibits a mass fraction of greater than 5% ofdextrin molecules with a molecular weight of greater than 1,000,000 Da.5. The coating colour according to claim 1, wherein the dextrincomprises, with respect to its total weight: a mass fraction of between5 and 25% of dextrin molecules with a molecular weight of greater than1,000,000 Da, a mass fraction of between 25 and 50% of dextrin moleculeswith a molecular weight of greater than 100,000 Da and less than orequal to 1,000,000 Da, a mass fraction of between 30 and 50% of dextrinmolecules with a molecular weight of greater than 10,000 Da and lessthan or equal to 100,000 Da, a mass fraction of less than 20% of dextrinmolecules with a molecular weight of less than or equal to 10,000 Da. 6.The coating colour according to claim 1, further comprising anadditional polymer conventionally used as binder in coating colours,said polymer being selected from the group consisting of styrenepolymers, polymers of (meth)acrylic or of (meth)acrylate, saponified ornonsaponified polyvinyl acetates, protein binders, and mixtures thereof.7. The coating colour according to claim 1, wherein the coating colourcomprises, per 100 parts of pigments, from 1 to 25 parts of dextrin andfrom 0 to 10 parts of additional polymer, the sum of the parts ofdextrin and additional polymer ranging from 4 to
 30. 8. The coatingcolour according to claim 7, wherein the coating colour comprises, per100 parts of pigments, from 2 to 15 parts of dextrin and from 2 to 7parts of additional polymer, the sum of the parts of dextrin andadditional polymer ranging from 6 to 25 parts.
 9. The coating colouraccording to claim 1, wherein the coating colour exhibits a pH rangingfrom 8 to 10.5.
 10. The coating colour according to claim 1, the solidscontent of which is between 40 and 85%.
 11. The coating colour accordingto claim 1, wherein the coating colour exhibits a Brookfield viscosityranging from 500 to 2000 mPa·s and/or a Hercules viscosity ranging from40 to 80 MPa·s.
 12. A process for preparing a coating colour accordingto claim 1, comprising: a step of mixing the water, the pigment, thedextrin and the optional additional polymer; a step of recovering thecoating colour; wherein the mixing step is carried out at a temperatureranging from 5 to 50° C.
 13. The process for coating paper, furthercomprising at least one step of coating a paper with the coating colouraccording to claim
 1. 14. A coated paper obtained from the processaccording to claim
 13. 15. The coated paper according to claim 14,wherein the coated paper comprises from 1 to 5 layers, at least one ofsaid layers being obtained from the coating colour.
 16. A process forprinting a coated paper according to claim 14, comprising printing isthat of offset, heliogravure, rotogravure, lithogravure, ink jet orflexography type.